1. Field of the Invention
The present invention relates to mobile communication, and especially to a method for supporting a relocation of an IP session during a network layer handover in a mobile communication system, and a mobile node and a network node supporting the method.
2. Description of the Related Art
A mobile communications system refers generally to any telecommunications system wherein the access point to the system may change when users move within the service area of the system. The mobile communications network is, correspondingly, an access network providing an end user with wireless access to external networks, hosts, or services offered by specific service providers. The service area of the system may comprise different access technologies and several administrative domains.
The new mobile communication systems have been developed to facilitate widespread use of new applications, also including ones that require more bandwidth and extended transmission sessions compared to earlier technologies. On the other hand, the ubiquitous coverage of current cellular systems has led the end users to expect similar availability of services from the next generations of systems. Therefore, seamless service provisioning for the considerable range of different applications will be a critical issue for the success of the new mobile communication systems.
In the context of providing wireless access using the Internet Protocol (IP), seamless IP layer mobility refers to the ability to hand over a mobile node (MN) to a new access router (AR) with minimal disruption to the IP connectivity. In the auspices of the Internet Engineering Task Force (IETF), a number of solutions for seamless IP layer mobility have been generated. Mobile IP, as defined in Request for Comments (RFC) 2002, is an enhancement of the Internet Protocol version 4 (IPv4) that adds mechanisms for forwarding Internet traffic to mobile nodes when they are connecting through a network other than their home network. Similar mechanisms have been developed for Internet Protocol version 6, referred to as IPv6. Each mobile node is assigned a permanent home address on its home network and a care-of address that identifies the current location of the device within a network and its subnets. Each time a mobile node moves to a different network, it acquires a new care-of address. A mobility agent (also known as Home Agent) on the home network associates each permanent address with its care-of address.
As an enhancement to this, fast handover protocol allows a mobile node to configure a new care-of-address before it moves towards a new subnetwork with the aim of being able to use it directly after its connection to the new access router. Consequently, the latency time is minimized and potential loss of packets during handoff is effectively eliminated.
In the process of establishing the new forwarding path for IP flows, mere creation of connection to the new nodes, however, might not be enough. The nodes along the new path must be prepared to provide similar forwarding treatment to the IP packets. This is especially important for services with particular requirements, such as time sensitive VoIP telephony and video and streaming services, whose successful employment in mobile environment depends heavily upon the ability to minimize the impact of the traffic redirections. A context transfer procedure is a specified method, which aims at provisioning of seamless IP layer connectivity. Context relates to the information transferred from one network entity to another as a means of re-establishing routing related services on a new subnet or a group of subnets. Context transfer thus facilitates seamless transfer of the mobile node's (also known as mobile terminal, station or device) packet session a the new access router while the session can be re-established without having to perform the entire protocol exchange between the new node and the mobile node.
In order to perform fast handover and context transfer procedures as described above, the Candidate Access Router Discovery (CARD) as described in the IETF Seamoby Working Group Internet Drafts “Candidate Access Router Discovery” of October 2002, and “A Dynamic Protocol for Candidate Access Router Discovery” of October 2002, provides means for discovering the IP addresses of the potential next access routers, and such characteristics of the access routers that may be of interest to an MN when the access router is evaluated as a handover candidate. Through this potential next access router discovery (CARD), at the time of the IP layer handover the potential next access router whose capabilities appropriately match with the requirements of the mobile node may be selected as a target access router. For enhancing the established CARD solution, a protocol for maintaining and updating the information on capabilities of the neighboring access routers in each of the access routers has also been proposed in the prior art.
However, even though the presented mechanisms allow the mobile node to be able to immediately exchange packets with the new network node and even transfer a session to a new access router without interruption, there are cases where the mobile node may still not be able to continue the service without disruption after the handoff. This is due to the fact that the existing solutions are designed to reveal the existence of a requested capability in the new access router, but they do not disclose whether the pre-discovered resource is available to the transferable session at the time of the handoff. Such temporary lack of appropriate resources is imminent, however, whenever the application requires a specific functionality of a network node, and the successful execution of the application functionality does not allow for breaks in the data transfer.
For example, let us consider a user of a mobile node MN1 moving along a road and utilizing a streaming application with a specific bandwidth. Through the specified potential next access router discovery the capability of the selected target access router to support said bandwidth may be verified. However, before the initiation of the network layer handover of the mobile node MN1, another mobile node MN2 may have already been handed off to the selected target access router, and the resulting available bandwidth in the access router is lower than what is necessary for a successful continuation of the ongoing session in the mobile node MN1. The result of such a situation is degradation or even teardown of the session of MN1 at handover.
As another example, let us consider a user of a mobile node moving along a certain road, and crossing a sequence of access routers and administrative domains. Somewhere along this route the user may also need to cross a technology boundary from 2 G to 3 G, which means that a specific transcoding functionality is needed because of the different bandwidth capabilities of the traversed networks. However, it is possible that at the time of the actual handover the transcoding functionality is no longer available for the mobile node. It is also possible that the discovery of the transcoding element may take too much time for the relocation to happen without disruption. In such a case, the handoff will severely disrupt the active service.
In view of the above, in addition to the comprehensive measures for IP layer mobility and connectivity, a solution for enhancing the seamless relocation of an IP session of a mobile node during a network layer handover is desirable.